The use of small-angle electron scattering to compare the structure of craze found in thin films with that found in bulk materials

Small-angle electron scattering (SAES) has been used to examine the structure of crazes in polystyrene. It has been shown theoretically that the analysis of SAES is similar to the equivalent x-ray patterns (SAXS) except perhaps for the higher-angle scattering. A direct comparison of the SAES patterns from crazes in films of ca. 400 nm thickness with SAXS patterns from crazes in films of ca. 1 mm thickness has shown that the craze structures are similar in form for the thin and thick films but the fibrils are about three times larger in the thin films.     

Craze fibril structure and coalescence by low-angle electron diffraction

Brown has shown that low-angle electron diffraction (LAED) may be used to determine fibril diameters D and spacings D₀ of crazes in thin polymer films. He found, however, that the D and D₀ determined for air crazes in polystyrene (PS) thin films were larger by about a factor of 3 than those in PS bulk crazes determined by using small-angle x-ray scattering (SAXS). We have repeated Brown's LAED experiments and find that the discrepancy may be caused by an aging effect. Our fresh crazes have D and D₀ values from LAED that are comparable to those of bulk PS crazes determined by SAXS. As the craze ages, however, fibrils retract and coalesce in wide regions of the craze, leading eventually to an observable “skin.” Aged crazes thus have much larger D and D₀ values than do fresh crazes. The large molecular mobility of the PS molecules in the fibrils necessary for this aging to occur at room temperature has important implications for fibril failure.     

Studies of orientation and structure of crazed matter in polystyrene. II. Electron diffraction measurements

The technique of selected area electron diffraction in the transmission electron microscope is used to examine the degree of orientation of crazed matter. The theories of electron and x-ray diffraction are compared and it is shown experimentally, by comparison with published x-ray results, that it is possible to obtain electron diffraction patterns from uncrazed polystyrene that are reasonably free of both radiation damage and multiple scattering problems. Electron diffraction patterns from crazes show a considerable degree of orientation but otherwise are very similar to those from uncrazed material, showing that crazes have no structure different from that of bulk material. Diffraction patterns are also obtained from thin films drawn to draw ratios of 4.5 and 6 at 90°C. These agree well with published x-ray results from oriented polystyrene but show less anisotropy than the craze diffraction patterns.     

Microvoid formation during deformation of high-impact polystyrene

Small-angle x-ray scattering (SAXS) has been used to study the formation of microvoids in polymers which craze or stress-whiten extensively. Specimens are subjected to a stepwise uniaxial strain, with scattering curves being obtained at each step. The increase in scattering intensity upon crazing is attributed to the formation of microvoids, and the relative size, shape, and concentration of the scattering elements are determined by a Porod analysis of the SAXS curves. The major portion of our work has been on high-impact polystyrene which shows a large increase in SAXS intensity as crazing occurs. We are able to follow the changes in void size and concentration during craze initiation and growth. Effects of temperature, molecular orientation, and matrix molecular weight have also been studied. The results add to the information on craze growth and microstructure known from electron microscopy and dilatometry. In addition, a qualitative physical model for microvoid nucleation is proposed, and the implications for toughness are discussed.     

An analysis of the reflective component of small-angle X-ray scattering patterns from crazed polystyrene

An investigation of the reflective component of small-angle x-ray scattering patterns of polystyrene crazes is performed. It is shown that the strong streak parallel to the tensile axis consists predominantly of reflected radiation, while the remaining pattern is composed entirely of diffracted radiation. X-ray reflection off unparallel regions of crazes is discussed and the nature of the reflection is also considered in terms of collimation quality. An analysis methodology based on craze tip angular distribution is proposed.     

Low-angle x-ray scattering from crazes and fracture surfaces in polystyrene

Both crazes and fracture surfaces in glassy polymers produce a low-angle scattering of x-rays. Scattering patterns are anisotropic and often show considerable streaking. In the one case (polystyrene) studied extensively so far, detalied analysis suggests that the craze is approximated as a collection of spheroidal or irregular-shaped voids surrounded by material with anisotropic density distribution arising from its orientation in the stress direction. The void dimension is about 90–115 Å and the specific internal surface area about 170 m²/cm³ of craze. These results and those from electron microscopic studies are reasonably consistent.     

New treatment of the theory for small-angle x-ray scattering with applications to polystyrene crazes

The phenomenon of surface scattering of electromagnetic waves by single or multiple layers of films is reviewed and a special treatment for the total reflection of x rays is developed. This theory is applied to the analysis of the surface scattering observed in small-angle x-ray scattering (SAXS) studies of two-phase matter in polymers having lamella stacks or a flat interfacial boundary structure. Important features of this vector theory are the ability to calculate the surface scattering invariant, the absolute scattering intensity, and the surface roughness, which gives rise to dispersion of specular reflection from perfectly smooth surfaces. By considering the interfacial surface roughness of polystyrene crazes, the surface scattering spectrum is calculated theoretically and compared with some experimental results. Also the theory is presented in such a way as to compare surface scattering with volume scattering; i.e., both two- and three-dimensional scattering events can be simultaneously treated. This provides a new basis for quantitative analysis of crazes in polystyrene.     

Development of crazes in polycarbonate,investigated by ultra small angle X-ray scattering of synchrotron radiation

The development of crazes in polycarbonate is investigated with the method of ultra small angle X-ray scattering of synchrotron radiation. Measurements at T = 130°C are discussed. The two-dimensional scattering patterns are analysed by means of a simple fibrillar model of the crazes. The geometrical parameters of the crazes as a function of the macroscopic draw ratio λ_d are determined using a curve-fitting procedure. The craze fibril volume fraction ν_f shows a complex dependence on λ_d.     

The optical interference pattern in a polystyrene craze layer

In the interference pattern seen in monochromatic light reflected from a craze layer in polystyrene, the bright fringes are of alternating intensity. The phenomenon is explained in terms of a four-beam interference. Apart from the two reflections from the outer surfaces of the craze layer others arise from a thin layer of approximately constant thickness within the craze layer at its median plane. The phenomenon provides independent evidence to support the electron microscopic observations of the microstructure of crazes in polystyrene.     

Crazing in thin films of styrene–butadiene–styrene block copolymers

The mechanism of craze initiation and growth and its relationship to mechanical properties has been studied in thin films of styrene–butadiene–styrene (SBS) block copolymers. Optical microscopy and transmission electron microscopy were used to examine three copolymers which has a spherical rubber domain morphology but varied in rubber content from 20 to 50%. With increasing rubber content, the crazes became longer and less numerous. Widening of the crazes was at least partially responsible for the higher strains achieved in the copolymers, especially for the composition with the highest rubber content where the crazes widened to form micronecks. Transmission electron microscopy revealed that craze initiation and growth at the craze tip occurred by cavitation in the polystyrene phase. Cavitation of the continuous phase rather than the rubber domains was attributed to the concentration of chain-end flaws in the polystyrene. Crazes in the block copolymers followed a meandering pathway and the boundaries between crazed and uncrazed material were indistinct. Incorporation of fibrillated rubber particles into the craze fibrils strengthened the craze. At higher rubber content, the craze widened in the stress direction by voiding and fibrillation, which produced a cellular morphology.     

A SAXS study of a single crack and craze in plasticized polystyrene

Small-angle x-ray scattering (SAXS) was used to study the structure along a single craze that had broken down to form a crack along part of its length. This study was made possible by use of radiation from the synchrotron source CHESS which is sufficiently intense to permit examination of just a single craze. The total scattering from the craze was in excellent agreement with that expected from a knowledge of its dimensions and fibril volume fraction and width. This fact adds confidence to the interpretation of the scattering pattern of the craze as part diffraction, part reflection, and demonstrates that SAXS is a technique that may be used to measure craze volume within a sample. The craze was shown to grow in width by surface drawing with a constant structure, and then the fibrils broke to form a crack. The broken fibrils contracted and their diameters increased but they appeared to stay parallel with a constant fibril-axis radial distribution function.     

Ar atom emission as a probe of craze formation and craze growth in polystyrene

A report of measurements of Ar emission during the loading of polystyrene and high impact polystyrene in vacuum is presented. Argon was introduced into the material prior to the experiment by storing the samples in an Ar atmosphere. The development of crazes during loading was monitored by videotaped visual observations and scattered light measurements. Increased Ar emission is observed at the onset of crazing, provided that the crazes intersect the surface. The strength of the Ar signal depends upon the extent of crazing; especially intense signals are observed from samples which display significant crazing prior to fracture. High-impact polystyrene shows intense emissions at yield which soon decay due to the depletion of Ar from the near surface material. The emission intensity rises again prior to fracture, when surface crazes become connected to crazes in the bulk. Thus the emission of volatile species during deformation reflects the growth of crazes intersecting the surface, as well as changes in the “connectivity” of the craze network. © 1993 John Wiley & Sons, Inc.     

Studies of orientation and structure of crazed matter in polystyrene. I. Optical measurements

The optical birefringence and refractive index have been measured for crazes grown in specimens of varying thickness. The birefringence was found to consist of the sum of a small negative orientation term and a large positive form term. The latter term could be altered by filling the craze with liquids of various different refractive indexes. Two crazes, which showed a relatively constant width as the specimen thickness changed, could be described by a model with craze having a constant refractive index and birefringence but with impervious dense skins on either side. The numerical value of the form birefringence was approximately 0.6 of that predicted from a model of parallel rods which is not surprising as crazes have a networklike structure. The values of craze refractive index were in good agreement with other measurements. The existence of skins, of thickness approximately 300 nm, places some doubt on the relevance of thin-film electron microscope observations to the situation in the bulk. Other crazes which tapered in width were found to show both varying skin thickness and refractive index along their length.     

The effect of plasticization on craze microstructure

The structure of crazes in plasticized polystyrene has been studied by means of small-angle x-ray scattering and optical interference microscopy. Addition of plasticizer causes a rapid increase in the mean fibril diameter D and a slow decrease in the craze fibril volume fraction v_f. The crazing stress σ_c was also measured and it was found that the product D σ_c is independent of plasticizer concentration. These results are shown to be consistent with the entanglement model for controlling v_f and the meniscus instability model of craze thickness growth.     

The entanglement network and craze micromechanics in glassy polymers

Crazes have been grown from crack tips in thin films of the following five polymers: polytertbutylstyrene (PTBS), polystyrene (PS), poly(styrene-acrylonitrile) (PSAN), poly(phenylene oxide) (PPO), and poly(styrene-methyl methacrylate) (PSMMA). These polymers represent a wide range of l_e values, where l_e is the chain contour length between entanglements. Quantitative transmission electron microscopy has been used to analyze the extension ratio λ_craze and displacement profiles for these crazes. From these measurements the craze surface stresses have been computed by using the method of distributed dislocations. This analysis also permits an accurate measure of the level of the applied stress σ_∞. These measurements show that the stress necessary for crazing increases as l_e decreases and that the higher surface stresses present at crack tips generate crazes that have higher λs than isolated crazes in the same polymers. Surface drawing is shown to be the dominant mechanism for craze thickening in all five polymers.     

高抗冲聚苯乙烯中银纹的引发与终止

银纹是由孔穴和断裂面间相联结的原纤维组成的微小裂纹,其中原纤维的体积分数可达40%.银纹的体积分数与材料的韧性成正比.银纹化是高抗冲聚苯乙烯(ＨＩＰＳ)在脆化温度以下,抵抗破坏而消耗外界能量的主要方式.银纹的产生与材料内部不均一性所导致的应力集中有关.ＨＩＰＳ中的橡胶粒子能够控制银纹在本体中均匀地发展,这是ＨＩＰＳ高韧性的原因[1].ＨＩＰＳ的分散相是由聚丁二烯(ＰＢ)为连续相,ＰＳ为分散相构成的细胞结构粒子.通常ＨＩＰＳ中ＰＢ的含量为7%～8%,而细胞结构粒子的体积分数可高达23%,可见细胞结构粒子内部ＰＳ的含量为ＰＢ的…     

Electron microscopy of crazes in glassy polymers: Use of reinforcing impregnants during microtomy

Attempts to prepare undamaged microtomed sections of crazes without reinforcement have failed. Several methods of reinforcing crazes in glassy polymers with impregnants prior to microtomy have been tried. Generalized characteristics of successful impregnant systems are suggested on the basis of this experience. The most successful system has involved the infusion of liquid sulfur into crazes in poly(2,6-dimethyl-1,4-phenylene oxide). After quenching, the solid sulfur reinforces the crazes successfully during microtomy but subsequently sublimes away under vacuum. The resultant, largely undamaged craze structure is seen by transmission electron microscopy to resemble an open-cell foam, the holes and polymer elements of which uniformly average ～200 Å in diameter. A moderate degree of orientation in the original tensile stress direction is observed. Implications drawn from craze structure for the existence of order in the glassy state are discussed.     

DYNAMIC MECHANICAL RESPONSE OF CRAZES IN POLYSTYRENE

Dynamic mechanical analysis was used to study the mechanical properties and microstructureof crazes in polystyrene produced in air or in methanol at different temperatures. A new loss peakwas found at about 82℃,which is assigned to glass transition peak of craze fibrils. The decreaseof glass transition temperature of polymer in craze fibrils is due to the high values of surface tovolume ratio. The glass transition temperature ratio of craze fibrils to bulk material (T_g~l /Tg) hasbeen expressed as a function of the fibrils diameter (d). From T_g~l of craze fibrils,the value of fibrildiameter can be calculated. Annealing the crazed specimen at room temperature makes the fibrilsplastically deform and cause the fibrils to thin slightly,whereas annealing the crazed specimen atthe temperature near T_g of the craze fibrils makes the fibrils bundle together.     

Electron microscopic investigations of initiation and growth of crazes in polystyrene

The crazes in polystyrene (PS) were investigated by using a high voltage electron microscope (HVEM, accelerating voltage of 1000 kV). The early stages of the formation and the growth of the crazes were studied in detail.The smallest deformation structures visible are weak domains or microvoids with diameters of 10–15 nm and distances of a few 10 nm between them. They act as craze nuclei and are located in narrow, long pre-craze zones. Conclusions are drawn on the processes of initiation and propagation of the crazes; both are based on molecular heterogeneities and on an increasing heterogeneity of deformation. In particular, the transformation of the closed cell structure of the voids into the open cell structure of the craze fibrils is described.Growth of crazes in thickness definitely occurs by drawing new polymeric material from the craze boundaries into the craze.     

Mechanical properties of methanol crazes in poly(methyl methacrylate)

Methanol crazes are grown from sharp cracks in poly(methyl methacrylate) (PMMA). The craze thickness profile is measured using a replica technique after the craze opening displacement profile of the growing craze has been measured with holographic interferometry. The craze strain profile is then computed from these data. The craze surface stress profile is determined by two methods: (1) from the uniaxial strain profile of regions adjacent to the craze as measured from the fringe spacing on the reconstructed hologram and (2) from the craze opening displacement profile using the Fourier transform method of Sneddon. From the surface stress and craze-strain profiles a true stress-strain curve for the craze fibrils has been constructed. The extrapolated fibril yield stress is in good agreement with the yield stress of bulk PMMA plasticized with methanol indicating that surface tension effects do not contribute importantly to craze fibril mechanical properties at room temperature. The craze strain increases from 0.4 near the craze tip to 1.4 near the craze base implying that methanol crazes in PMMA thicken by further straining of the existing craze fibrils and not by drawing new material into the craze from the craze surfaces. The primordial craze thickness, i.e., the original thickness of polymer which fibrillates to form the craze fibrils, is approximately 1 μm and is constant over most of the craze length. This thickness may be determined by diffusion of methanol normal to the craze surfaces in a process zone just behind the craze tip.